Excimer lasers including MOPO systems and master oscillator power amplifier (MOPA) systems are extensively used as light sources for UV microlithography in the manufacture of state-of-the-art semiconductor integrated circuits. Such excimer lasers have to satisfy challenging and often mutually conflicting technical requirements, due to an ever-increasing demand for smaller critical dimensions of the integrated circuits, together with higher production throughput and reduced running cost of UV microlithography systems. Important requirements of an excimer laser for UV microlithography include high output power and pulse energy, long output pulse duration, narrow spectral bandwidth and high spectral purity of the output beam, and high reliability and long lifetime of components.
One reason for the long pulse requirement is that optical damage induced in the projection lens of a UV microlithography system (step-and-repeat system or “stepper”), depends on the peak intensity of laser light in light-pulses delivered by the excimer laser. Reducing peak intensity while still maintaining average power in the pulses can extend the lifetime of a projection lens without reducing system throughput. Since average power, pulse repetition rate and beam size in the projection lens are properties of the laser and can not be changed easily, peak intensity is typically reduced by extending the duration of a laser pulse after it is delivered by the laser, and before it is delivered to the projection lens. This is accomplished through the use of what is known in the art as a passive pulse-stretcher, the pulse stretcher being located at the laser output.
Such a passive pulse-stretcher typically consists of a beam splitter and several mirrors for beam folding and imaging. The mirrors form an optical delay line. Some of the light in a pulse input to the pulse stretcher is transmitted through the beam splitter, and some of the light is reflected. The reflected light is sent through the delay line and, after one round trip through the delay line, returns to the beam splitter. There, some of the light is transmitted, while some of the light is reflected and sent through the delay line again. Such a pulse stretcher consequently generates a train of pulses, each thereof having a fraction of the energy of the originally input pulse, and which are separated temporally by the round trip time of the delay line.
One limitation the passive pulse-stretcher is that it is energy inefficient. This is due to scatter and absorption losses at the beamsplitter and components of the optical delay line. Optical losses can be as high as ten percent per round trip or even greater. Another limitation is that the stretching ratio of the pulse is limited. This is because the energy of pulses in the train generated by the pulse-stretcher becomes vanishingly smaller after the second pulse. This limits the stretching factor of the pulse-stretcher to less than 3. In a case where the pulse has to be stretched by a greater factor, two or more consecutive pulse-stretchers have to be used, which adds to energy inefficiency. Energy inefficiency converts to higher cost of the system and higher operating cost of the system. For these reasons, it would be preferable that the laser initially emitted a longer pulse, so that the pulse stretcher could be made simpler and more efficient.
High spectral purity, or low ASE level in the output of the laser, are important in order to achieve high contrast of the mask image at the wafer. This allows for reduced critical dimensions and increased process latitude. Typically, an integral ASE level of below several times 10−4 is required in systems operating at a wavelength of 193 nm.
High output power is important for high throughput of the manufacturing process. Typically, the required power is set by the transmission of the optical lithography projection apparatus and required rate of wafer throughput of the stepper. A conventional, single-oscillator, excimer laser cannot provide the required high output power with the required narrow spectral emission bandwidth. These two requirements are in conflict with each other, since for obtaining the narrow bandwidth the oscillator must run at low output power. At higher powers, sophisticated means for spectral line narrowing including prisms and gratings cannot be employed. Accordingly, preferred laser systems employ above-mentioned MOPA or MOPO laser systems. Such systems include a combination of a low-power master oscillator (MO) with superior narrow spectral emission bandwidth, and a power amplifier (PA) or a power oscillator (PO), which then amplifies a narrow bandwidth signal from the master oscillator to a high power output beam.
FIGS. 1A and 1B schematically illustrate prior-art MOPA laser systems 20 and 22 respectively. MOPA 20 includes a master oscillator 24 and a power amplifier 26. Master oscillator 24 includes a line-narrowing unit 25 including wavelength selective optical elements such as prisms, and often a grating used in cooperation with prisms. As such line-narrowing arrangements in a master oscillator are well known in the art, the arrangements are not discussed in detail herein. An output beam (pulse) 28 from oscillator 20 is directed by mirrors 30 and 32 into power amplifier 26. The beam makes a single pass through amplifier 26, is amplified therein, and is output therefrom as amplified radiation. In MOPA 22, a power amplifier 27 is arranged for double pass amplification. An output beam (pulse) 28 from oscillator 20 is directed by mirrors 30 and 32 into power amplifier 26. Beam 28 makes a first pass through amplifier 27 and is reflected from a mirror 31 back through the power amplifier. The beam is amplified on each pass through the amplifier and the amplified beam is directed out of MOPA 22 by mirror 33.
FIGS. 2A and 2B schematically illustrate prior-art MOPO laser systems 34 and 40 respectively. MOPO 34 includes a master oscillator 24 (including line-narrowing elements 25) and a power oscillator 36 having a stable resonator formed by a partially transparent input mirror 38 and a partially transmitting outcoupling mirror 39. An output pulse 28 from master oscillator 24 is directed by mirrors 30 and 32 through partially transmitting mirror 38 into the power oscillator and seeds the oscillator. This causes the power oscillator to oscillate with the characteristics of the seed pulse. MOPO 40 utilizes a Cassegrain-type unstable resonator power oscillator 37 formed between a concave mirror 41 and a convex outcoupling mirror 43. Mirror 41 has an aperture 42 therein which allows efficient incoupling of the seed beam (pulse) 28 into the power oscillator. In this type of power oscillator, mirrors 41 and 43 are usually arranged as close to the gain chamber of the oscillator (not explicitly shown) as possible. This results in a short round-trip time, for example about 10 nanoseconds (ns), which helps to achieve maximum energy extraction efficiency from the gain chamber, but the resulting output pulse length is only 20 ns. Having a shortest possible resonator is consistent with the limitations of space available within the volume and footprint of the laser enclosure, as both are very valuable in a clean room environment.
The output power of a MOPA depends directly on the power of the master oscillator. The optical design of a MOPA is simpler than that of a MOPO, at least when the power amplifier is used in single pass as depicted in FIG. 1A. However, the power extraction efficiency is limited, especially if the master oscillator emits low power. For a multiple pass amplifier, such as illustrated in FIG. 1B, the setup quickly becomes complicated, as the output and input beams have to be spatially separated. Since the output power of the amplifier depends on the input from the master oscillator, the master oscillator still has to emit comparably high power, which sets a limit to narrowing of the spectral emission bandwidth in the master oscillator.
In a MOPO arrangement, the master oscillator functions only as a low power seed source for the power oscillator, which oscillates and emits high power with the same bandwidth as the master oscillator. The power oscillator is a fully functional oscillator, which, in free lasing, emits maximum output power stored in the gain medium. The power of the seed source (master oscillator) has limited influence on the output power from the power oscillator. Since only low power is needed for seeding, the master oscillator can run at a much lower output power level. Because of this sophisticated methods for narrowing the spectral emission bandwidth can be employed. A MOPO arrangement, accordingly, is strongly preferred for high power narrow bandwidth operation.
In a MOPO system, however, degradation of internal optical components due to exposure to high energy UV radiation occurs more rapidly than in a MOPA laser. This is especially true of the outcoupling mirror of a MOPO laser, which is at normal incidence to a high-intensity output beam. Further, the power oscillator of a MOPO laser, if not seeded, oscillates independently and emits light with broad spectral characteristics. Also, if seeding is insufficient, a background of broad emission characteristics will overlay the narrow bandwidth spectrum of the seed source. Such background is highly undesirable, especially for lithography applications. Even when seeding is nearly perfect, some background of amplified spontaneous emission (ASE) still exists. Such background has to be suppressed to obtain high spectral purity.
Another disadvantage of a MOPO laser system results from cross talk between the master oscillator and the power amplifier. This cross talk can degrade the performance of the master oscillator. Seeding of the power amplifier is usually accomplished by means of a partially reflecting mirror, which is also the rear mirror of the power oscillator, as discussed above with respect to MOPO 34 of FIG. 2A. Both the seed beam and the rear mirror have to be aligned to the axis of the power oscillator. The master oscillator and power oscillator share the same optical axis, and the rear mirrors and outcoupling mirrors of both oscillators are normal to the optical axis. Cross talk results from mutual feed back between the two oscillators.
Two solutions have been applied to this cross talk problem. One solution is to provide sufficient optical length between the two oscillators that the entire length of the seed pulse is already emitted from the master oscillator before the cross talk can set in. Providing an optical delay line of sufficient length between master oscillator and power oscillator can effectively suppress cross talk. Such a delay line, however, consumes valuable space inside a laser enclosure.
Another potential solution would be to align the mirrors of the power oscillator off the optical axis of the master oscillator. This option, unfortunately, is not feasible for a linear power oscillator. There is a need to overcome the cross-talk problem and other above-discussed shortcomings of MOPO lasers.